Why does a slider gain not change during execution of Simulink Real-Time (Matlab 2014a)?

In a Simulink model which I run on an xPC Target (Simulink Real-Time, External mode) I have a slider gain. When I change its value during runtime this shows no effect. However, if I have it on a value > 0 and change the constant which is multiplied with the slider gain, the target implements the new value. This occurs now working with Matlab 2014a. In a model I used already with Matlab 2012b the slider gain worked during runtime in that older version but not in 2014a anymore.

• Is this a bug or was there changed something in this function on purpose?
• If this has to be, what's the use of the slider gain, then?
• If not, does anyone have an idea where to look for the reason of this problem?